DE 37 43 315 A1 discloses oxygen sensors mounted in an exhaust pipe used to continuously determine the air/fuel ratio with a high response rate in both the “lean”—lambda greater than one—and the “rich”—lambda less than one—mixture range. These, what are termed continuous or linear lambda probes, operate according to the two-cell limit-current probe principle and can be used as pre-cat probes for injection controlling (lambda controlling), but especially for controlling lean-burn engines, for example Otto engines having direct fuel injection.
The measuring signal of a lambda probe depends on a plurality of variables, including the oxygen concentration to be determined in the exhaust and the temperature of the ceramic and the counter-pressure of the exhaust, with the degree of pressure dependence of the measuring signal being defined by the design of the probe. A distinction is made where the pressure dependence is concerned between a static and a dynamic pressure dependence. Typical variations in the dynamic pressure dependence of the measuring signal are within the significant range in the case of continuous lambda probes and hence are an order of magnitude higher than for what are termed binary lambda probes. The following concerns the damping or, as the case may be, elimination of periodic pressure-related influencing factors, especially in connection with continuous lambda probes.
Pressure pulsations in the exhaust system are due partly to the abrupt rise in the positive pressure curve triggered by a pressure surge produced when discharge valves of a cylinder are opened. A waveform pressure curve is produced by reflections or overlapping exhaust oscillation in the exhaust system until another pressure surge occurs upon the cylinder's next ejection stroke. An internal combustion engine operated by the four-stroke method therefore produces a dynamic exhaust-pressure curve having a periodicity of 720° KW referred to the crankshaft, which is to say dependent on engine speed. The possibility of hardware-based filtering is limited, because the frequency of the pressure-dependent interference within the lambda signal depends on the internal combustion engine's speed, and the central control device of the internal combustion engine must remain suitable for measuring rapid processes (lambda controlling on a cylinder-selective basis, for instance). Based on the above-described characteristic periodicity of the processes, signal filtering requires averaging over a specific crank-angle range of the internal combustion engine, for example, in the case of a four-cylinder four-stroke internal combustion engine having a single-flow exhaust system, 720° KW/4=180° KW.
The generic method accordingly proposes an integration period or summation period corresponding to the engine speed dependent period of oscillation of the pressure curve, which is 180° KW in the example above. Above-cited DE 37 43 315 A1 mentions the possibility of providing separate summation equipment to relieve the vehicle's microcomputer of the special function of signal filtering. The following problems are involved:
The known method for averaging obviously requires a relatively large amount of memory to be reserved for the individual measurements of the lambda probe signal which are sampled in, for example, a 1-ms time-slot pattern and buffered in a ring memory. For further processing of the lambda probe signal, averaging would then be initiated at each instant at which a filtered output signal is required (e.g., every 10 ms) by totaling a number N1 of buffered individual values and dividing the result by N1. For the given sampling time-slot pattern the number N1 would exactly correspond to the period of oscillation of the pressure curve. With this procedure, for a four-cylinder internal combustion engine, 50 individual values would have to be stored simultaneously in the memory for, e.g., 600 revolutions; for a 6-cylinder two-bank system a total of 67*2=134 individual values would have to be stored. Averaging would furthermore always, which is to say at each update time, have to be carried out across the entire number of N1 measurements for the period to be considered so that, especially in the case of slow engine speeds, the summation value would be formed several times over certain sections of the memory.